shm_open failing on read only file system

I'm trying to define a shared memory for IPC across two processes on an embedded device where the root is readonly.

It appears that this condition does not allow the creation.

How can I create a shared memory on RO filesystem?

This is the incriminated code:

//...
auto m_fmap = shm_open("/common_memory", O_CREAT | O_RDWR | O_EXCL, (S_IRUSR | S_IWUSR));
    if (m_fmap  == SHARED_INVALID) {
        printf("error opening sh. mem!\n");
        Close();
        return false;
    }